1. Field of the Invention
The invention relates to curable dispersions applied as protective coatings to optical wave-guides. After curing at elevated temperature, these coatings allow passage of actinic radiation used to modify optical waveguide transmission characteristics. More particularly the present invention provides coating compositions comprising finely divided particles dispersed in a polysiloxane to yield thermally cured write-through coatings having transparency to both visible and ultraviolet radiation to facilitate changes of refractive index in an optical fiber that may be modified to include fiber optic gratings including dispersion compensation gratings.
2. Description of the Related Art
Manufacturing processes for high purity glass optical fibers typically include in-line coating equipment to apply protective polymeric coatings to fibers drawn from a melt or solid preform. A glass fiber, as drawn, exhibits very high tensile strength. Flaws develop on the surface of an optical fiber during contact with solids and many liquids. This leads to undesirable weakening of the optical fiber. A protective coating, applied before contact of the fiber with either contaminants or solid surfaces, aids retention of inherent high strength as it protects the fiber.
Conventional processes for incorporating light modifying features into coated optical fibers require removal of protective coatings from manufactured optical fiber structures. The coatings typically attenuate passage of ultraviolet radiation. Exposure of coated optical fibers to high intensity ultraviolet radiation for through-coat variation of refractive index generally causes coating decomposition and deterioration of beam intensity reaching the optical fiber core. Modification of light transmission characteristics of optical fibers is desirable to include a variety of special features in selected, relatively short lengths of optical fibers to be spliced or otherwise incorporated into fiber optic systems and devices. A fiber Bragg grating represents a light-modifying feature that may be introduced or written into an optical fiber by exposure to ultraviolet radiation. Gratings may be written for a variety of applications including dispersion compensation, controlling the wavelength of laser light, and modifying the gain of optical fiber amplifiers.
A capability for through-coat refractive index variation of optical fibers would overcome the need to remove protective coatings before modifying the core of an optical fiber. Use of a substantially radiation-transparent or write-through coating also moderates the need to apply a protective recoat material after exposing a coated optical fiber to ultraviolet radiation. Elimination of process steps contributes to production efficiencies at lower cost.
Write-through coatings for optical fibers have been described for a variety of polymer types including fluorinated polymers and polysiloxane materials. Claesson et al (International Wire & Cable Symposium Proceedings 1997, Pages 82-85 (46th Philadelphia, Pa.)) use two polymers to coat germanosilicate optical fibers prior to exposure to an ultraviolet radiation pattern to produce Bragg gratings in optical fibers so exposed through the polymer coatings. The coatings, applied by solvent dip or die draw, were TEFLON AF 1600 and KYNAR 7201. When thin (20 μm-50 μm) films of KYNAR 7201 were exposed to a pulsed excimer pumped frequency doubled dye laser at a wavelength of 242 nm, the plastic rapidly degraded, darkened and decomposed.
No degradation was observed for films (6 μm) of TEFLON AF 1600 coated on boron co-doped fibers during exposure to a pulsed excimer pumped frequency doubled dye-laser at 242 nm to write a Bragg grating (1 cm long) using an interferometric technique. The estimated fluence in the core per pulse was 1 J/cm2 and the accumulated dose for writing the grating was 140 J/cm2. Optical fibers were coated using relatively crude conditions including the use of a fluorosilane and heating to 330° C. for 10-15 minutes to improve adhesion.
Imamura et al (Electronics Letters, Vol. 34, No. 10, pp. 1016-1017) describes the preparation of a coated optical fiber and conditions used to expose the fiber to ultraviolet radiation during writing of a Bragg grating. The ultraviolet radiation source was a frequency quadrupled Q-switched YAG laser operating at 266 nm. This laser was capable of delivering a mean power of 100 mW at 10 Hz repetition with pulse duration of 50 ns. The description includes further detail of conditions used to form a Bragg grating.
The only information regarding the fiber coating material describes it as an ultraviolet curable resin formulated with a photoinitiator for increased transparency at 266 nm. Recommended conditions for forming a Bragg grating through a 60 μm coating of the resin include 10 minutes exposure to a dose of 150 J/cm2. At this condition the ultraviolet absorbance at 266 nm wavelength was <1.07.
Chao et al (Electronics Letters, Vol. 35, No. 11 (27 May 1999) and U.S. Pat. No. 6,240,224) discusses drawbacks of earlier attempts to write gratings through coatings over optical fibers before discussing the use of a thermally cured silicone coating (RTV 615). This material has suitable transparency to ultraviolet radiation since it contains no photoinitiator that would attenuate ultraviolet beam intensity. An ultraviolet spectrum reveals that a 150 μm thick layer of silicone between silica plates will transmit 85% of incident radiation at a wavelength of 225 nm. From 225 nm to 235 nm and above there is a gradual increase of radiation transmitted to 92%. Low absorption of ultraviolet radiation offers the possibility of Bragg grating writing through the silicone rubber coating using either a frequency doubled Argon-ion laser at 244 nm or a KrF excimer laser at 248 nm.
Although omitting both the coating steps and conditions, a patent to Aspell et al (U.S. Pat. No. 5,620,495) describes formation of an optical fiber grating by writing through a methylsilsesquioxane coating. Organosilsesquioxane coatings are known to undergo significant shrinkage when they cure. Bagley et al (U.S. Pat. No. 4,835,057), for example, describes glass fibers having organosilsesquioxane coatings that fail to protect the underlying optical fiber core when used as coating layers that are less than 5 μm thick. Honjo et al (U.S. Pat. No. 5,052,779) describes organosilsesquioxanes as ladder-type polysiloxanes having low elongation. Low elongation leads to cracking during curing of coatings made from these materials. According to the reference, the cracking problem may be reduced when the coating formulation contains a linear polymethyl siloxane having hydroxyl groups and solvent in addition to the ladder-type polymer. Bautista et al (U.S. Pat. No. 4,962,067; EP 902067 and EP 1123955) describe the effect of viscosity variation on the properties of coatings containing ladder-type siloxane polymers.
Transparent coatings, as described above, are known as write-through coatings. Chao et al (Electronics Letters, Vol. 35, No. 11 (27 May 1999) and U.S. Pat. No. 6,240,224) in fact recommends the use of thermally cured silicone coatings as candidate materials for write-through coatings. Application of thermally cured silicones to optical fibers retains maximum ultraviolet transparency by avoiding the use of compositional components that may absorb ultraviolet radiation. Absorption of radiation during periodic modification of the refractive index of an optical fiber interferes with formation of a refractive index grating in the fiber.
Claesson et al (International Wire & Cable Symposium Proceedings 1997, Pages 82-85 (46th Philadelphia, Pa.)) describes the use of fluorinated polymers as write-through coatings. Imamura et al (Electronics Letters, Vol. 34, No. 10, pp. 1016-1017) discusses photocurable resins including photoinitiators having minimal absorption in a portion of the ultraviolet spectrum. These write-through resins were not identified. Other omissions from previous descriptions include the use of continuous processes for applying write-through coatings and the conditions and amount of time required to cure selected coatings circumferentially around the fiber. Such omissions reinforce the need for improvement in coating compositions and methods for applying write-through coatings to optical fibers so as to improve the production rate for fiber optic devices including refractive index gratings also referred to as Bragg gratings.